Determination of w-value for particle beams in several gases

The differential w-values were measured for proton, deuteron, 3He ion, neutron and 60Co gamma-ray beams. The measurements were made by eight ionization chambers with different shapes and sizes. The gases investigated were air, argon, nitrogen, carbon dioxide, methane and methane based tissue-equivalent gas, and the density of the gases at 0degC, 101.3kPa is 1.293, 1.784, 1.250, 1.977, 0.717 and 1.142mg/cm3, respectively. The tank gases were used of pure grade. The gas flow rate was approximately 10ml/min. For proton, deuteron and 3He ion beams, a monitor ionization chamber constructed with very thin plates was located in front of the exit window. Nitrogen was with a flow rate of 40ml/min was used as the ionized gas and -1000V was applied on the high voltage plate. For neutron beams, The target current on the Beryllium was used as a beam monitor. The differential w-values were determined from the Bragg-Gray cavity theory. For the 70MeV proton beams, we estimated the w-value of 35.3, 27.0, 36.5, 34.3, 27.9 and 30.4eV in the gases mentioned above, respectively. Our results agree with those recommended in ICRU report 31. Similar results were also obtained for the other particle beams. The use of these w-values for the evaluation of absorbed dose with ionization chambers for proton beams gave good agreement with a tissue-equivalent calorimeter. (author)